1. Field of the Invention
The present invention relates to arrays of light-emitting elements on monolithic devices and more particularly to such arrays that incorporate as part of the monolithic device a light sensor for sensing output from the elements.
2. Description of the Prior Art
The term "monolithic device" as used herein refers to a device that is formed on a single semiconductor crystal chip array. Light-emitting diode (LED) chip arrays are well known in the prior art and are typically used in printheads for electrophotographic copiers or the like. For example, see U.S. Pat. No. 5,253,934, the contents of which are incorporated by this reference. Such a printhead array comprises a row of uniformly spaced LED light sources that can be individually energized to generate light to expose a photoreceptor or other image receiving medium to produce an image pattern. A typical LED printhead for this type for standard DIN A4 paper dimensions would be about 216 millimeters long and the individual light sources are very small and very closely spaced, e.g., as many as 400 or more sites per linear inch (15.75 sites per mm). A complete printhead comprises a number of individual LED chip arrays, each being typically less than 10 millimeters long, which are mounted in endwise relation to one another in forming of the full length printhead.
High-density, linear LED chip arrays can be fabricated as a monolithic device to be used as light sources in electronic printing application. With existing technology, it is possible to fabricate an array of light-emitting diodes on a single gallium arsenide chip which, without further correction, is adequate for two-level exposure. A two-level exposure is produced when an LED can selectively be turned on or off. LED chip arrays with a large number of exposure levels are required for good quality, continuous-tone images. At any given exposure level, any one of the LEDs should produce light with an intensity that is within an acceptable range. For example, the light intensity of a diode at a given exposure level may be required to be within a few tenths of a percent of the light intensity at the same exposure level of any other array light-emitting diode on the printhead. If such is not the case, undesirable bands may appear in the output image.
The output image intensity of LEDs fabricated in a monolithic device and driven at constant current degrades nonuniformly with usage. In order to prevent undesirable bands, it is necessary to provide dynamic correction, i.e., the output light intensity at each exposure level of each LED of a chip array must be periodically measured and corrected so that across the entire array, each exposure level is within the proper intensity range.
In order to provide this dynamic correction, it is known from U.S. Pat. No. 4,588,883, filed in the name of Abbas, to incorporate on the monolithic device a photodetector that can sense subsurface light generated by each LED which subsurface light is related to that emitted by the LED. The Abbas patent describes a single, wide, monolithic photodetector fabricated along one length of the LED arrays. In practice, high density (.gtoreq.400 dpi) LED printheads containing LED arrays have wirebonds from the control electronics (Driver ICs) accessing the LED arrays from both sides. This results in addressing the odd/even LED channels of each LED chip from opposite ends. To implement Abbas' invention in such dual-sided addressed LED writers would require an additional process step during the LED wafer fabrication, to run LED electrode/bond pad metallization traces over the photodetector towards the Driver IC wirebonds.
It is therefore an object of the invention to eliminate this extra process step by yet providing a monolithic device having an array of photodetector devices in already available real estate on the LED chip surface.